Yeah, this is a good question and I'm not completely sure where I land on it. On one hand, there was a kind of flowering of non-D&D RPGs in the 90s when D&D was weakest (or market fragmentation, depending on your perspective). On the other hand, you could argue there's been a second flowering of non-D&D RPGs during this period when it's been at it's strongest. I do think what happened in the 90s strained 1990s distribution channels (that's the market fragmentation argument), but I wouldn't mind seeing what a "weak D&D" period looks like in the Kickstarter era.
Despite that, in the real world we live in, I'd don't think we're about to enter a "weak D&D" period. All signs point the other direction, for the time being.